Abstract: Prostate cancer (PCa) is the second leading cause of cancer-related deaths in American males. Androgen deprivation therapy or blockage of androgen at the level of androgen receptor (AR) remains the treatment options for advanced disease. However, emergence of androgen resistance limits their therapeutic effectiveness and most patients with PCa progress from androgen-dependent status to invasive castration- resistant PCa (CRPCa) for which there is no effective therapy. CRPCa continues to express AR and depends on functional AR signaling for growth. Studies have demonstrated that c-FLIP, an anti-apoptotic protein, reduces the efficacy of AR-targeted therapy. More importantly, the expression of c-FLIP correlates with progression to CRPCa. c-FLIP blocks apoptosis either by competing with caspase 8 to bind death effector domains of the adaptor protein FADD or by averting the recruitment of caspase 8 to FADD. Ku70, a crucial component of the homologous end-joining DNA repair machinery, interacts with c-FLIP and regulates its protein stability by inhibiting polyubiquitination. Therefore, finding agents that can disrupt AR function, especially in conjunction with other small molecules that target c-FLIP, could dramatically improve the outcomes of therapeutic interventions for CRPCa. We found that fisetin is a potent inhibitor of AR signaling and interacts with the ligand binding domain of AR. In addition, fisetin treatment reduced the invasive potential of PCa. Furthermore, treatment of PCa cells with vorinostat increased the acetylation of Ku70 and decreased c- FLIP expression. The combination of fisetin and vorinostat more effectively reduced c-FLIP protein expression and increased PARP cleavage when compared to individual agents. Therefore, this application proposes to focus on the preclinical assessment of a rationally based combination against CRPCa cells by testing the anti- androgenic and anti-invasive activities of fisetin in combination with vorinostat that particularly target HDAC6 and increases the acetylation of Ku70 by disrupting c-FLIP/Ku70 complex leading to c-FLIP degradation and induction of apoptosis. The hypothesis to be tested is that ?fisetin in combination with vorinostat will synergistically delay the onset of castration-resistant tumor progression and would prevent metastasis by reducing invasion and To test this hypothesis, two specific aims are proposed: (i) to examine the effects of fisetin in combination with vorinostat on cell growth, invasion and apoptosis of androgen-depleted and CRPCa cells, and (ii) to investigate the effects of fisetin in combination with vorinostat on PCa development and metastasis in castrated Pten knockout mice. This proposal aims to utilize a non-toxic dietary agent, fisetin, in combination with vorinostat for the treatment of CRPCa cells, which are a major cause of mortality and morbidity associated with advanced stage PCa. We believe that successful completion of this proposal would lead to the development of an extremely valuable and novel therapeutic approach for treatment of CRPCa. apoptotic elimination of CRPCa cells?.